Pulp screening apparatus



June 19M, E. CZQWAN 2246,59

PULP SCREENING APPARATUS Filed June 15, 1938 s Sheets-Sheet 1 IN VENTUOH ELIJAH COWAN AT TO HNEY June 24 19% 1 QQWAN 2,246fiw PULP SCREENING APPARATUS Filed June 13, 1958 3 Sheets-Sheet 2 INVE N TOH/ ELIJAH.GOWAN ATTORNEY June 24, 1941. cow I 2,246,669

. PULP SCREENING APPARATUS Filed June 15, 1938 s Sheets-Sheet a FIG. maxim @fiwmlW ATTORNEY rsieniedfiunez i, 1941 UNITED s11.n'r1;:s PATENT,

FFICEi I rm scnnnnmc'arranarus Elijah Cowan, Dolbeau, Quebec, Canada Application in... 1a, 1938, Serial No. 213,459

4 Claims. This invention relates to pulp screening apparatus and has for its object the provision of a generally improved and more eflicient centrifugal pulp screen of the Quiller type.

In the operation of a conventional Quiller" typescreen the stock is supplied to the inlet under suitable head pressure and flows-axially along the annular passage between the impeller shaft and the cylindrical screenuntil it reaches the cone end of the'impeller where the flow is changed from axial to radial. The radial flow ,is accelerated by the centrifugal force imparted to the pulp suspension by the revolving propeller blades. so that the water and fine fibres are forced outwardly through the perforations of the cylindrical screen. The coarse fibres are not well hydrated and, due, to their lower specific gravity, tend'to lag behind the finer fibres at the cone part of the impeller. As these coarse fibres accumulate they gradually work out of the cone part of the impeller and form a mat on the adjacent inner surface of the screen. This mat. which usually contains a considerable percentage or the desirable finer fibres, gradually moves along the screen toward the coarse fibre outlet. At the tailings end of the screen dilution water is introduced under pressure 'through shower pipes by which jets of the water are directed against the screen to thin out the mat so that thefiner fibres are washed out through the perforations ofthe screen. This method of introducing dilution water does not give sumcient penetration and is also open to the objection that it occasions considerable shock losses as the impeller blades pick up the'water jets directed through the shower pines.- According to the present invention these objections are oversome by conducting the dilution water throu h the impeller shaft to radiallv extending grooves formed on .the pressure sides "of the impeller blades. The dilution ,water introduced in this manner need not be .under any considerable presalong the line 4-4 of Fig. 1.

sure since the centrifugal force. to which the v water is subjected as itienters the grooves of the impeller blades is suffic ent to throw the water toward the screen with the r clu site velocity to 1 ensure efiicient diiut onof the fibres. in addition to thinnin out the mat formedon the ad J'imentinner surface of the 'screen.

I have also found that, owing to the improved penetration obtained, the delivery of the dilu-v tion water to grooves formed in the pressure sides of the impeller blades materially increases the capacity of the screen in the zone where the along the portion of dilution water is introduced to thin out the fibrous mat.

The invention further comprises a screen of the Quillerf type in which the impeller ispro vided, near the cone end, with baflles which serve,

by their fiow retarding effect, to ensuregcom plete filling of the cylindrical screenat the inlet or cone end of the impeller. This ensures a more uniform screening action along the entire length of the screen than is obtained in the conventional screen, in which the pulp supplied through the inlet tends to flow rapidly toward the coarse fibre outlet without filling the screen Proceeding now to a more detailed description t0 the of this invention reference will be had accompanying drawings, in whichn Fig. 1 is a vertical sectional view of one form of screening apparatus embodying my invention. Fig. 2 is an end view of the impellerand shaft assembly shown in Fig. 1. w I

Fig. 3 is a transverse sectional view through the impe ler andv shaft assembly. the plane of the section being along the line 3-3 ,of Fig. 1.

Fig. 4 is. atransverse sectional view of the impeller and shaft assembly taken Fig. 5 is a view similar to Fig. 1, but showing a slight modification. V Fi 6 is a transverse sectional view, along the line 8 B of Fig. 5. p

The; screening apparatus shown in Figures 1 to 4 inclusive comprises an outer casing 5. a cylindrical screen 8. and a rotary impeller].

Casing 5 maybe of any suitable of" preferred design. In the present instance it comprises a base section 8, end heads 9 and I 0. and removable cover plates ll.v The base is recessed at 811. to receive the lower portions of the end heads which are fastened inplace by bolts II. The cover plates II are curved to fit the rim portions of the end. heads and are bolted or otherwise detachably secured in place. The end porscreen at the cone substantially t-ions of screen 6 are held in place between the annular ribs 13 of the end heads 9 and i and suitable retaining strips M. The end head ill is provided with a relatively large central opening l5 to take the feed elbow I5 which is fastened in place as indicated at H.

During the screening of the pulp suspension supplied through elbow IS the oversized particles gradually move along the screen 6 toward the end head 9 and are discharged through the outlet IS. The finer material which passes through the screen is discharged through an opening [9 formed in the base 8. passes centrally through the screen 6 and is iournalled in suitable bearing structures 2| and 22 carried by the end head 9 and the feedelbow IS. A driving pulley23 is fixed to shaft adjacent bearing 2| and is driven by a belt 24.' The belt 24 also passes over a tightening pulley 25 carried by one end of a lever 26. The remaining end of lever 26 is mounted to swing-about a pivot pin 21 carried between the end head 9 and the cover plate 28 of bearing 2|. Any suitable means may be provided for operating pulley 26 to tighten the belt 24.

Shaft 20 is cored out to provide a water passage 29 which extends approximately one-half the length of the shaft. This passage extends inwardly from the pulley end of the shaft and is closed at its inner end as indicated at 30.

During operation ofthe screen dilution water is continuously supplied to passage 29 through a water supply pipe (not shown) suitably connected to the pulley end of shaft 20. The wall of the water passage 29 is provided with suitable openings 3l registering with similar openings 32 formed in the hollow hub 33 of the impeller I. The openings 32 are in open communication with the inner ends of radially. extending grooves 35 formed in the pressure sides of the impeller 'vanes 36 which have a slight helical twist and are cast integral with the hub 33.

The cone end 34 of impeller hub 33 is spaced inwardly from the end head Ill of casing 5. Portions of the impeller blades 36 project beyond this end of the hub into close proximity with the end head l9 but are recessed or cut away as indicated at 39 to provide a vane-free space 4 into which the inner end of the feed elbow l6 extends. The ends of the impeller blades 10 The impeller shaft 29' cated adjacent the end'lhead ID are connected together by an integral strengthening ring 4|.

Bafiles 42 are also cast integral with intermediate portions of the hub 33 and the impeller blades 36. These baffles serve, by their flow retarding effect, .to ensure completefilling of the space lying between the baffles and the end head ID. This ensures a more uniform screening action along the entire length of the screen 8 than wouldbe possible if the baflles were omitted. In the latter event the pulp supplied through the elbow l6 would flow toward the outlet It! without filling the space immediately adjacent the end head [0. Consequently, but little effective screening action would be obtained along the portion of the screen immediately adjacent the inner end of the feed elbow.

The recessed portions of the impeller blades 36 are also interconnected by integral dam forming members 45. These dam members 45 lie substantially midway between the ring 4| and the bafiles 42. During the screening operation the blades'of the impeller impart a centrifugal force to the pulp and thus serve to force the fibre and water through the perforations of the screen 64 Owing to insufficient dilution the coarse particles of the pulp suspension tend to lag behind since the forces acting on them are smaller due to their lower specific gravity. As these coarse fibres accumulate they move toward the inner surface of the screen 6 and form a mat in the zone lying between the bafl'ies 42 and the end heads I0. This mat, which contains a considerable amount of desirable finer fibres, tends to move slowly along the screen toward the outlet I 8.

The dilution water supplied to the grooves 35 of the impeller blades 36 is thrown against the mat as it travels past the grooved portions of the blade. Inthis way the mat is thinned out sufflciently to permit the desirable fibres to be washed out through the screen openings. The purpose of the dams 45 is to prevent a too rapid travel of the mat toward the outlet l8 and to provide an increased depth of mat between the dam members and the bafiies 42. Increasing the depth of the mat between the dams 45 and they bafiles 42 increases the capacity of the, screen in this area due to the increased centrifugal force acting on the fluid of the pulp suspension.

The introduction of the dilution water through the water passage 29, openings 3| and 32, and grooves 35 eliminates the shock losses characteristic of pulp screens in which the water is sprayed into the screening chamber in advance of the travelling impeller blades. With the arrangement described herein the water is supplied to the grooved pressure sides of the blades without shock and is thrown against the screen in an efiicient manner to thin out the mat and wash out the desirable fibres. It may also be pointed out that the water suppliedto passage 29, need not be under any appreciable pressure since the centrifugal force to which the water is subjected as it enters the grooves 35 is sufiicient to cause it to flow radially toward the screen with the requisite velocity to ensure efficient dilution of in. The portions of the impeller blades extending from'the baffles 42 to the end head I 0 are shaped so that their inner edges are spaced from the shaft 20 and are interconnected by integral walls 41 forming the annular chamber 48 surrounding said shaft. Each of. the walls 41 are provided with slots 49 forming radial outlets for the pulp suspension 7 supplied to chamber 48 through feed elbow I6. I have found that this construction also gives an improved screening effeet along the portion of the screen lying adjacent the end head 10. In all other respects the assembly shown in Figs 5 and 6-is substantially the same as that previously described in connection with the preceding figures. l The vanes shown in Figs. 5 and 6 may b straight lined vanes or may be helically twisted in the same manner as the vanes shown in Figs. 1 to 4 inclusive.

Having thus described what I now conceive to be the preferred embodiment of this invention it will be understood that various modifications may be resorted to within the scope and spirit of the appended claims.

Having thus described my invention, what I claim is: x

1. Pulp screening apparatus of the Quiller type comprising a stationary cylindrical screen through which the pulp suspension is passed, an impeller comprising a plurality of impeller blades fixed to a hub mounted on a rotary impeller shaft extending axially through said screen, said shaft being provided with an internal water passage having radial outlet openings communicating with radial openings formed in said hub and each of said impeller blades being provided with radial grooves formed in the pressure side thereof and having their inner ends communicating with certain of the hub openings and means for supplying dilution water to said passage for delivery to said grooves.

2. Pulp screening apparatus as set forth in claim 1 including a casing enclosing said screen, said casing being provided with a pulp inletand a coarse fibre outlet located at opposite ends of the screen and battles interconnecting intermediate portions of the impeller blades to retard the flow of pulp toward the coarse fibre outlet.

3. Pulp screening apparatus as set forth in claim 1 including a casing enclosing said screen, said casing being provided with a pulp inlet and a coarse fibre outlet located at opposite ends of the screen, bailies interconnecting intermediate portions of the impeller blades to retard the flow of pulp toward the coarse fibre outlet, and dam forming members interconnecting the impeller blades at points located between the baflles and the pulp inlet, said dam forming members being arranged to regulate the travel and thickness of the fibrous mat forming on the surface 0! the screen at the inlet side of the baiiles.

4. Pulp screening apparatus as set forth in claim 1 including a casing enclosing said screen, said casing being provided with a pulp inlet and a coarse fibre outlet located at opposite ends of said screen, baflies interconnecting intermediate portions of the impeller blades to retard the flow of pulp toward the coarse fibre outlet and axially extending wall members interconnecting said blades at the side of the bailies facing the pulp inlet, said axially extending wall members serving, in conjunction with the impellenshaft and the baiiies, to provide a plurality of chambers into which the pulp is delivered through said inlet, said axially extending walls being apertured to provide outlets through which the pulp passes from said chambers into the surrounding spaces between the impeller blades.

ELIJAH COWAN. 

